docs: correct plugin example filename (#7026)

* Correect Wrong file name

The file name inside the my-plugin folder is index.js .

* Update plugins.md

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

website/docs/advanced/plugins.md

@@ -51,7 +51,7 @@ module.exports = {
 
 Then in the folder `my-plugin`, you can create an `index.js` such as this:
 
-```js title="my-plugin.js"
+```js title="my-plugin/index.js"
 module.exports = async function myPlugin(context, options) {
   // ...
   return {